The CFM is determined by the size and shape of the fan blade in conjunction with the RPM of the motor. That being said, the actual energy efficiency would be determined by dividing the CFM by the energy being used to produce the CFM.

How do you know how many watts a motor uses?

On a single phase motor you multiply the motor rated amps x the motor voltage will determine the amount of watts used by a motor. and on a three phase motor you multiply the wattage by 1.73

What is meant by CFM?

CFM means cubic feet of air per minute. It is the measurement of airflow.